1. Field of the Invention
The present invention relates generally to a mobile terminal and power control method for random access of the mobile terminal, and in particular, to a power control method and apparatus of a mobile terminal that facilitates a random access procedure in a distributed antenna mobile communication system.
2. Description of the Related Art
FIG. 1 illustrates a conventional cellular mobile communication system including three cells, each centered around a transmit/receive antenna, i.e., the transmit/receive antenna is located at the center of each cell. The cells are commonly referred to as a Central Antenna System (CAS). Even when multiple antennas are provided, all of these antennas are arranged at the center of the cell to define the service area.
Referring to FIG. 1, each of cells 100, 110, and 120 is centered around an antenna (or centrally located antennas) 130 associated with an evolved Node B (eNB). The eNB serves first and second User Equipment (UEs) 140 and 150 within cells 100, 110, and 120 to provide mobile communication service. Specifically, within cell 100, i.e., the service area of the eNB using the antenna 130, the first UE 140 is served at comparatively lower data rate than the second UE 150, because the first UE 140 is farther from the antenna 130 than the second UE 150.
In a mobile communication system implemented with the CAS-based antenna formation as illustrated in FIG. 1, each eNB transmits reference signals for a UE to measure a downlink channel state and modulate downlink signals. For 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A), the UE estimates channel with DeModulation Reference Signal (DM-RS) and measures a channel state between the eNB and the UE based on Channel Status Information Reference Signal (CSI-RS), the DM-RS and CSI-RS being transmitted by eNB.
FIG. 2 illustrates a conventional resource block including CSI-RSs transmitted by an eNB. Specifically, FIG. 2 illustrates a downlink reference signal structure with a DM-RS and a CSI-RS transmitted from an eNB to a UE in an LTE-A system.
Referring to FIG. 2, the x axis is the time axis, and the y axis is the frequency axis. A minimum transmission unit in the time domain is an Orthogonal Frequency Division Multiplxing (OFDM) symbol, and a subframe 224 includes two slots 222 and 223, each including NsymbolDL symbols. A minimum transmission unit in the frequency domain is a subcarrier, and the system frequency band is divided into a total of NBW subcarriers. A basic unit of a time-frequency resource is a Resource Element (RE), which is defined by an OFDM symbol index and a subcarrier index. A Resource Block (RB) 220 or 221 is defined with NsymbolDL contiguous OFDM symbols in the time domain and NSCRB contiguous subcarriers in the frequency domain. That is, one RB includes NsymbolDL x NSCRB REs. A minimum transmission unit of normal data or control information is a RB.
In FIG. 2, the downlink control channel is transmitted in the first three OFDM symbols at the beginning of the subframe 224. A Physical Downlink Share Channel (PDSCH) is transmitted on the remaining resources, after those allocated for the downlink control channel in the subframe. The DM-RS is the reference signal that is referenced by a UE to demodulate the PDSCH.
The RB of FIG. 2 is designed to transmit strings for two CSI-RS antenna powers at the positions denoted by reference numbers 200 to 219. Specifically, numbers 200 to 219 denote the positions paired for the signals of two CSI-RS antenna ports. Accordingly, the eNB transmits the downlink estimation signals for the two CSI-RS antenna ports at the position 200.
An antenna port is a logical concept, such that CSI-RS is logically defined per CSI-RS antenna port for channel status measurements of a respective CSI-RS antenna port. If the same CSI-RS is transmitted through multiple physical antennas, the UE cannot discriminate among the physical antennas but just recognizes a single antenna port.
In a mobile communication systems including a plurality of cells, as illustrates in FIG. 1, it is possible to transmit a CSI-RS at cell-specific location, as illustrated in FIG. 2.
For example, the CSI-RS can be transmitted at position 200 in cell 100, the CSI-RS is transmitted at position 205 in cell 110, and the CSI-RS can be transmitted at position 210 in cell 120. Basically, the cells are assigned different time-frequency resources for the CSI-RS in order to avoid interference between the CSI-RSs of different cells.
In a CAS as illustrated in FIG. 1, however, the antennas of each eNB are concentrated at the center of cells limiting the ENBs abilities to provide a high data rate service to a UE located far from the center of the cell.
FIG. 3 illustrates a conventional mobile communication system configured with both a CAS and a Distributed Antenna System (DAS).
Referring to FIG. 3, the mobile communication system includes cells 300, 310, and 320. As illustrated in more detail, the first cell 300 includes a central antenna 330 and four distributed antennas 360, 370, 380, and 390. The central antenna 330 and the distributed antennas 360, 370, 380, and 390 are connected each other and controlled by a central controller of an eNB.
The central antenna 330 provides mobile communication service to first and second UEs 340 and 350 located in the first cell 300. However, because the first UE 340 is located farther from the central antenna 330 than the second UE 350, the first UE 340 is served by the eNB at a comparatively lower data rate than the second UE 350.
Typically, as the propagation path of the signal elongates, received signal quality degrades. By deploying a plurality of distributed antennas 360, 370, 380, and 390 within the cell 300 and providing the first and second UEs 340 and 350 with the mobile communication service through the distributed antennas 360, 370, 380, and 390, selected according to the locations of the first and second UEs 340 and 350, it is possible to improve the data rate. For example, the first UE 340 communicates through distributed antenna 390, which provides the best channel environment for the first UE 340, and the second UE 350 communicates through distributed antenna 360, which provides the best channel environment for the second UE 350. Accordingly, each of the first and second UEs 340 and 350 may be served by the eNB at a high data rate.
Normally, the central antenna 330 supports normal mobile communication services, service not characterized as high speed data services, and the mobility of the first and second UEs 340 and 350 crossing the boundaries of the cells 300, 310, and 320. Each of the central and distributed antennas may include a plurality of antenna ports.
FIG. 4 illustrates a conventional mobile communication system configured with central antennas distributed throughout a cell.
Referring to FIG. 4, the mobile communication system includes a plurality of cells 400, 410, and 420, each cell including a plurality of central antennas 430, 431, 432, 433, and 434 distributed throughout the cell and a plurality of distributed antennas 460, 470, 480, and 490 distributed in the cell. The central antennas 430, 431, 432, 433, and 434 are provide first and second UEs 440 and 450 with normal mobile communication services, i.e., those not characterized as high speed data services, and support mobility of the first and second UEs 440 and 450 roaming across the cells 400, 410, and 420. The distributed antennas 460, 470, 480, and 490 provide high speed mobile communication services.
In the following description, the logical concepts of a Central antenna port (C-port) and a Distributed antenna port (D-port) are defined such that the central and distributed antennas can be discriminated logically from each other regardless of their physical configurations.
The C-port defines a CSI-RS to support CAS for each antenna port, such that a UE can measure a channel status for each antenna port of the C-port. The CSI-RS transmitted through the C-port covers an entire area of a cell.
The D-port defines a CSI-RS to support a DAS for each antenna port, such that a UE can measure a channel status for each antenna port of the D-port. The CSI-RS transmitted through the D-port covers a local area within the cell. However, if the same CSI-RS is transmitted through multiple antennas, the UE cannot discriminate between the antennas located at different positions, but instead identifies the same antenna port.
For example, in FIG. 3, if the third and fourth antennas 380 and 390, which are located far from each other, transmit CSI-RS #1 and CSI-RS #2, each having different patterns, the first UE 340 can measure the channel state between the third distributed antenna 380 and the first UE 340 based on the CSI-RS #1 and the channel state between the fourth distributed antenna 390 and the first UE 340 based on the CSI-RS #2. In this case, the third distributed antenna 380 is referred to as D-port #1, and the fourth distributed antenna 390 is referred to as D-port #2. If the third and fourth distributed antennas 380 and 390 transmit the CSI-RS #3 having the same pattern, the first UE 340 cannot discriminate between the third and fourth distributed antennas 380 and 390 using the CSI-RS #3. The first UE 340 measures the channel states between the first UE 340 and the distributed antennas 380 and 390 using CSI-RS #3. In this case, the combination of the third and fourth antennas 380 and 390 is referred to as a D-port #3.
The time-frequency resources for transmitting C-port CSI-RS and D-port CSI-RS are allocated so as not to overlap with each other, thereby avoiding interference.
In attempting an initial connection to the LTE-A system, a UE performs a cell search to acquire downlink timing and frequency synchronization and a cell IDentifier (ID). Thereafter, the UE acquires basic parameters related to communication, e.g., a system bandwidth in the system information transmitted by the eNB. The UE then performs a random access process to transition to a connected state on a link to an eNB.
FIG. 5 is a signal flow diagram illustrating a random access process in a conventional mobile communication system.
Referring to FIG. 5, a UE transmits a random access preamble to an eNB in step 501. The eNB measures the propagation delay between the UE and eNB and acquires uplink synchronization. The UE selects a random access preamble randomly in a given random access preamble set. The initial transmit power of the random access preamble is determined using a pathloss between the eNB and the UE, as measured by the UE.
In step 502, the eNB transmits a time alignment command to the UE based on the propagation delay measured in step 501. The eNB also transmits scheduling information including uplink resource information and a power control command. If no scheduling information (a random access response) is received from the eNB, the UE repeats step 501.
In step 503, the UE receives uplink data (message 3) including a UE ID to the eNB using the uplink resource allocated in step 502. The transmit timing and transmit power of the UE is determined according to the command received from the eNB in step 502.
In step 504, if the eNB determines that the UE has performed the random access process without collision with other UEs, the eNB transmits, to the UE, the data (message 4) including the ID of the UE. When the message 4 is received from the eNB, the UE determines that the random access has completed successfully. When the random access has completed successfully, the UE configures an initial transmit power of an uplink data channel and/or control channel based on the UE transmit power controlled through the random access.
However, if message 3 transmitted by the UE, e.g., collides with data transmitted by another UE, such that the eNB fails receiving the message 3, the eNB stops transmitting data. Further, if message 4 is not received within a predetermined time, the UE determines that the random access has failed, and then repeats step 501.